The U.S. Synthetic Fuels Corp. has approved an infusion of $720 million in new federal subsidies for the country's largest synthetic fuels plant as part of a complex refinancing package aimed at keeping the project's corporate sponsors from shutting it down.

In a 4-to-1 vote announced yesterday, the corporation gave conditional approval to new price guarantees for the problem-plagued Great Plains coal-gasification plant in Beulah, N.D., along with a rescheduling of the project's $1.5 billion debt to the federal government.

The decision, which must also be approved by the Energy Department, was assailed by the Synfuels Corp.'s critics on Capitol Hill.

The project, sponsored by a consortium of energy companies led by Tenneco Inc. and American Natural Resources Co., now produces synthetic natural gas from coal at prices that are well above the market price. As declining energy prices have made the project less feasible economically, the companies have repeatedly said they would be forced to abandon the plant unless they received new subsidies.

"What we're doing is giving them a helping hand," said Thomas J. Corcoran, vice chairman of the agency's board of directors. "What you have here is an entity that is near bankruptcy -- it's a technical success but a business disaster. But the question is: do we walk away from it today and leave the government a $1.5 billion loan debt to swallow? . . .

"We don't want them to go out of business," Corcoran added. "We think it's in the public interest for the country to get the knowledge and experience in synthetic fuels from this project."

Yet board Chairman Edward E. Noble, who has long championed the project, voted against the proposal, saying in a prepared statement: "I do think the country needs this project, but not at any cost, and I would have preferred more time to consider the full deal before any formal vote of the board."

Sen. Howard M. Metzenbaum (D-Ohio) called the vote "an abominable abdication of government responsibility" that appears to "increase rather than decrease federal exposure."

As part of the new refinancing package, the Great Plains sponsors will be permitted to default temporarily on their loan payments and $673 million of the project's debt will be restructured. At the same time, they will begin repaying the remaining $850 million on an accelerated schedule with funds generated from the new subsidies and current tax breaks.

Assuming the project makes money -- a prospect critics say is increasingly remote -- the $673 million in restructured debt would be paid back under a schedule that would be stretched out to the year 2008.

Meanwhile, the $720 million in price subsidies will provide Great Plains a guaranteed price for its natural gas of $6.75 per million BTUs, more than a dollar above the $5.65 it now gets and more than twice the current market price of $3 per million BTUs.

The agreement also gives both the Great Plains sponsors and the Synthetic Fuels Corp. the right to terminate the deal on Aug. 1, 1986 or Aug. 1, 1987 in the event of an adverse ruling by the Federal Energy Regulatory Commission that would make it more difficult to pass on the cost of the higher-priced natural gas to consumers.

Corcoran said that a Great Plains bargaining team had agreed to the new terms in negotiations Monday night that included Energy Department officials. But Tom Haan, a Great Plains spokesman, declined to comment on whether the new assistance would be sufficient to keep the project alive. The companies had originally sought $820 million in price guarantees.

"I have no idea if this is an adequate deal or not," said Haan. "We won't know until we do a lot more staff analysis."