Western diplomats here said today they do not expect a major clash between Syrian and Israeli forces in neighboring Lebanon as the withdrawal of Palestinian guerrillas from Beirut winds down to an end this week.

For a variety of reasons of self-interest, intricately linked to Lebanon's domestic tranquility, the diplomats said, it will not serve the immediate design of either Israel or Syria to engage in hostilities now despite their belligerent posturing.

There was no response here today to the shooting down of a Syrian Mig25 over Lebanon. High-level Syrian officials have turned down repeated requests for interviews, and Syria has made no recent public comment on the future of its occupation forces in Lebanon.

Israeli Prime Minister Menachem Begin has repeatedly emphasized that the second goal of the Israeli invasion of Lebanon is the withdrawal of several thousand remaining Palestine Liberation Organization guerrillas and the 30,000 Syrian troops from east and north Lebanon.

Syrian President Hafez Assad has just as often reiterated that Syrian troops will not be withdrawn from Lebanon under the threat of the militarily stronger Israelis.

Knowledgeable senior diplomats here said that while Israel has the military strength to push the Syrians out, the costs in men and material would be very high for both sides.

The major flashpoint remains Lebanon's Bekaa Valley, where both armies have been heavily reinforced in recent weeks. The valley is Syria's first line of defense before the capital, Damascus.

Among Western diplomats, who said they have had extensive discussions among themselves on the subject, there is a general consensus "that a lengthy stalemate will develop." A stalemate is likely, one diplomat said, while Israel tries to achieve other goals of the invasion such as a permanent peace with Lebanon.

Israel sees its future relations with Lebanon as closely tied to the fortunes of Lebanese President-elect Bashir Gemayel, a Christian Phalangist militia commander who faces considerable domestic Moslem opposition and Syrian antipathy. Gemayel is scheduled to take office on Sept. 23, and diplomats here do not expect any major Israeli moves before then that could jeopardize a smooth presidential transition.

The Syrians already have been irritated, however, by continued references by Israeli leaders to the possibility of a peace treaty with Lebanon. Christian political aides to Gemayel have been shuttling between Beirut and Damascus trying to neutralize Syrian opposition to him.

The Syrians have considerable influence among Lebanese Moslems and would use it against Gemayel if they become convinced he favors a peace treaty with Israel, diplomatic sources agreed.

Before and since the election, Lebanese Moslem leaders visiting President Assad in Damascus have accused Gemayel of being put into office by "Israeli tanks," allegations that have been given prominent play in the tightly controlled Syrian media.

In recent days, Assad has publicly received two of Gemayel's most implacable domestic opponents, former prime minister Rashid Karame and former president Suleiman Franjieh. Diplomats see the meetings either as a warning to Gemayel not to go too far with the Israelis or as the typical Syrian practice of hedging all bets if an accommodation cannot be worked out with Gemayel.

While providing a platform for the criticism of Gemayel, the Syrian government has been careful not to directly criticize him since his election eight days ago, diplomats noted.

"The Syrians may have to work with Bashir [Gemayel] and are so far keeping their options open," said one ranking diplomat.

"If Bashir avoids a peace treaty with Israel, does not get into settling old scores [from the 1975-1976 Christian-Moslem Lebanese civil war] and brings a respected Moslem into the government as prime minister, the Syrians could work with him," the source said.