A small army of lawyers, corporate executives, environmentalists and politicians is expected to gather at the Federal Energy Regulatory Commission (FERC) this morning to argue about the project they call Iroquois, a proposed natural gas pipeline from the St. Lawrence River to Long Island.

The hearing will give the pipeline's many opponents a last shot at dissuading the FERC's four members from voting to authorize it, as they are scheduled to do at a July 25 meeting.

But the outcome may not be in doubt. A 99-page order approving the pipeline and rejecting every argument against it has already been written. And a required presidential certificate authorizing a connection to pipelines in Canada has been approved, according to FERC documents.

The order declares that the 364.4-mile, $583 million pipeline, which is to carry Canadian natural gas across New York and Connecticut, is needed to ensure adequate energy supplies in the Northeast and is environmentally acceptable. It rejects opponents' arguments that a full evidentiary hearing, where witnesses are cross-examined under oath, is required. And it rejects arguments that the proposed rate structure gives Canadian gas producers an unfair advantage over their U.S. competitors.

The order lacks only the commissioners' signatures to make it official and allow Iroquois' owners, a consortium led by TransCanada Pipelines Ltd., to begin acquiring right-of-way by eminent domain.

In addition to the order prepared for the commissioners' signatures, The Washington Post has obtained an extensive technical analysis of the Iroquois project by FERC's Office of Pipeline and Producer Regulation recommending approval. This document, dated June 29, was mailed anonymously in a franked FERC envelope. FERC spokesman Ron Harris confirmed its authenticity.

But he denied yesterday that the existence of the order means the commissioners have made up their minds. "There has been absolutely no decision on which way to go," he said. He said it was "standard operating procedure" for the FERC's senior staff to prepare a "draft order" in advance of a vote, and that the commissioners could decide after today's hearing to modify it.

The copy of the order made available to The Post is not stamped "draft," as it normally would be before becoming final, but Harris suggested that whoever leaked it erased that notation.

Representatives of the pipeline's opponents, still hoping to persuade the commissioners to reject it, declined to comment publicly on the order's significance. But two of them said privately it confirms their suspicion that today's hearing is a gesture to the critics, not a genuine attempt to obtain additional information. FERC has been under pressure from Senate Energy and Natural Resources Committee Chairman J. Bennett Johnston (D-La.), who has close ties to the domestic natural gas industry, to hold a full-scale hearing.

Iroquois' developers need their FERC certificate as soon as possible because their contracts with gas suppliers in Alberta will expire if gas is not flowing by Oct. 31, 1991. The contracts could be renegotiated, but the producers could demand higher prices. Robert Reid, president of Iroquois Gas Transmission Co., has said publicly that if FERC accedes to opponents' demands for an extensive judicial-type hearing, the delay could kill the project.

"We deny the requests for a trial-type hearing," the order says. "The commission is required to reach decisions on the basis of an oral, trial-type evidentiary record only if the material facts in dispute cannot be resolved on the basis of the written record." That is not the case with Iroquois, it says, rejecting as "inapposite" federal court precedents cited by opponents as requiring such a hearing.

Iroquois is supported by natural gas distributors in New England and by the state of New York, which needs more gas on energy-short Long Island. It is opposed by environmentalists who fear damage to wetlands and historic sites, by New England fuel oil dealers and by domestic gas producers.