Dr. David Dodge, the American educator kidnaped in Beirut a year ago and widely believed to be dead, returned to the United States in good health yesterday after a release almost as mysterious as his abduction.

A White House announcement yesterday morning, including praise for "the humanitarian efforts" of Syrian President Hafez Assad and his brother, security chief Rifaat Assad, in the case, was the first official word that Dodge was alive and had been released by his captors.

The surprise announcement came as Lebanese President Amin Gemayel and his aides sat down to meetings at the State Department focusing on the withdrawal of foreign forces from their country. Gemayel is to meet President Reagan today.

A U.S. military plane brought Dodge back to the United States yesterday from Frankfurt, West Germany, where he went from an undisclosed release site in the Middle East, according to U.S. sources.

Official Lebanese sources said that Dodge, at the time acting president of the American University in Beirut, had been spirited out of Lebanon following his abduction near the campus on July 19, 1982.

The captors were not identified in the White House statement, and officials would divulge only the sketchiest information, saying that Dodge has asked for maximum confidentiality and minimum exposure of himself and his case to the news media.

Diplomatic sources said Dodge had not been held in Syria, whose top officials reportedly had been making quiet efforts all year to obtain his release. Dodge was believed to have been captured by a pro-Iranian group of the Lebanese Shia Moslem community. There were unconfirmed reports last night that he had been held in Iran.

U.S. sources said no ransom had been paid nor other deals made to obtain Dodge's release. The Syrian officials are reported to have argued that Dodge, whose grandfather founded the American University in Beirut and whose father at one time was the university's president, had a family history and personal lifetime of service to the Middle East that deserved to be rewarded rather than punished.

A U.S. government physician found Dodge to be in good physical and mental shape during a preliminary, airborne examination following his release, the sources said.

Officials said the release was unrelated to the visit of Gemayel, who met for 3 1/2 hours yesterday with Secretary of State George P. Shultz.

The State Department session, which began in late morning and continued in a working lunch, was followed by meetings of U.S. and Lebanese aides in late afternoon on details of political-military issues and economic issues.

All the meetings were in preparation for today's White House discussion between Gemayel and Reagan and their senior assistants.

Gemayel, speaking at a breakfast sponsored by the National Press Club, sharply criticized the partial troop pullout from Lebanon approved early Wednesday by the Israeli cabinet, saying "this step could endanger all the peace process and also maybe the agreement of the 17th of May" between Israel and Lebanon for full withdrawal of Israeli forces.

"The partial withdrawal gives the impression that unfortunately, and I can't believe it, that the U.S. effort" to get all foreign troops out of Lebanon "has failed," Gemayel said. He added that a partial pullback by the Israelis would be seen by the Lebanese public as signaling a de facto partition of Lebanon, and would be exploited by "the enemies of the United States" in the Middle East.

Despite the outspoken criticism of Israel's announced plan, all indications are that Lebanon will accept it in practice and seek to move its army, possibly assisted by U.S. Marines and other elements of the multinational peacekeeping force, into areas evacuated by the Israelis.

U.S. sources said debate has been raging within the administration on a possible expansion of the role and size of the multinational force--currently 4,500 men from France, Italy, Britain and the United States, including 1,200 Marines.

The Defense Department, which was extremely reluctant to see U.S. forces assigned to Lebanon in the late summer and fall of 1982, is reported to be resisting the expansion of the force. Military officials have been taking the position that any increase in the Marines' mission or the area they cover would require assigning more Americans to Lebanon.

One plan discussed informally within the government calls for doubling the size of the multinational force, sources said.

The State Department, while cautious about any increase in the multinational force and especially in the U.S. component, is reported more willing than the Pentagon to contemplate an augmented role for the force in helping Lebanese army troops to take over from departing Israelis.

It was the State Department, especially U.S. special envoy Philip C. Habib, that insisted almost a year ago that a U.S. military presence was essential to arranging the negotiated withdrawal of Palestine Liberation Organization forces from Lebanon.

According to several accounts, Reagan appears less reluctant than the State Department to increase the U.S. force in Lebanon and notably less reluctant than the Defense Department. It is not clear, even to some officials involved in the intra-governmental discussions, why Reagan seems to have adopted a relatively activist view.