The federally subsidized Great Plains coal gasification plant, showpiece of the U.S. synthetic fuels industry, is likely to be a far more profitable investment for its owners than they are suggesting in seeking additional government aid, according to the General Accounting Office.

In a report to be released today, the GAO said that the five companies building the $2.1 billion plant in Beulah, N.D., appear to be in a position to "realize an average annual 20 percent return on their investment over the first 20 years the plant operates."

This contrasts sharply with the economic picture drawn earlier this year by Great Plains Gasification Associates, the consortium of five major energy companies building the nation's first commercial-size facility for production of synthetic natural gas from coal.

The consortium, promised as much as $2 billion in federally backed loans, said in March that instead of being able to earn a net profit of $1.2 billion in the first 10 years of operation as originally anticipated, it faces "losses in excess of $770 million."

Its change in outlook was blamed in the March report on the fact that the pricing formula approved by the government for synthetic gas to be produced by Great Plains is tied to the price of heating oil and the producer price index, which have not been rising at the rate they were when the project was launched.

As a result, Great Plains is expected to ask the government-funded Synthetic Fuels Corp. this week for hundreds of millions of dollars in price guarantees above its loan guarantee from the Energy Department, according to sources.

But the GAO study, which largely confirms findings of two recent independent studies of the viability of the project conducted for the Energy Department, seems certain to raise questions about whether Great Plains really needs additional federal aid.

The GAO said its conclusion that "the project's economics could be more optimistic than Great Plains' estimates" stems from the consortium's decision not to consider tax implications for its parent companies, which include huge firms such as American Natural Resources Co. and Tenneco Inc.

"By excluding these implications, Great Plains provides only a limited analysis of the project's financial viability," the GAO said.

"Although Great Plains estimates significant losses during the first eight years the plant operates, we found that there could be a positive cash flow to the partners throughout the life of the project if taxes are considered," the GAO reported.

The GAO said consortium representatives generally agreed that "the long-term economic viability of the project is attractive" but expressed concern "that their stockholders will not be willing to risk losses for eight years for the possibility of a more favorable return over a longer period."

"It sounds to us like GAO is saying, 'You have a shaky business situation, but you could shore it up by taking those tax benefits,' " a Great Plains spokesman said yesterday. "We believe that is terribly risky and the business should be on a solid basis to begin with."

Rep. Mike Synar (D-Okla.), chairman of the House Government Operations subcommittee that requested the GAO study, said yesterday that the report raises new questions about the "direction and timeliness of the entire synfuels program."

"The burden of proof clearly rests with the Great Plains' sponsors to justify any requests for more of the taxpayers' money," Synar said.

The GAO said it believes that the consortium, which expects to put up $517 million in equity while borrowing $1.55 billion in federally backed loans, is in a position to recover its full investment "within two years after the plant begins operating if after-tax cash flow is considered."

"During construction, the companies can take tax benefits for which they qualify on total project costs even though they contribute only 25 percent of the project's financing," the report said.

As a result, through a combination of investment tax credits, energy tax credits and interest deductions, the project could produce tax savings for the parent companies of $400 million during the construction phase, scheduled for completion in December, 1984, the GAO said.

"During 1984, the last year of construction, the potential tax benefits exceed the equity contributions the partners are expected to make," the GAO said. "The partners will contribute about $116 million in equity in 1984, but the parent companies' tax liability could be reduced by $155 million."

Then after the project begins operating, the consortium will qualify for "additional tax benefits such as depreciation," the GAO said. "Any losses that occur can be used to offset the parent companies' profits," it said.

The GAO said consideration of these tax benefits, assuming that the parent companies are profitable enough to take advantage of them, casts a different light on the project's viability.

"For example, the March, 1983, report shows that the partners would have to put $841 million into the project during the first eight years it operates," the GAO said. "During this same time period, however, the parent companies' tax liability could be reduced by $922 million."