Fears of an immediate jump in home heating costs have all but disappeared as the natural gas industry reaches a long awaited day of reckoning today when federal price controls are to be lifted from about half the nation's natural gas supply.

With the industry facing increased competition from alternative fuels, most government, consumer and industry analysts predict that only modest price rises will follow decontrol, and consumers in most parts of the nation should not see them in their fuel bills until spring or summer, if at all.

"Basically nothing will happen Jan. 1," said W. David Montgomery, a forecasting director at the U.S. Energy Information Agency. "The average person won't notice it."

"Natural gas prices are stable and are expected to stay that way," said Richard C. Vierbuchen, executive vice president of Washington Gas Light Co. "We think natural gas prices will remain relatively flat in 1985 or at most track an inflation rate of 4 or 5 percent."

This is a sharp reversal from last summer, when the EIA, an independent government fact-finding agency, predicted that residential fuel prices would rise 12 percent in the first quarter of 1985, and the Citizens-Labor Energy Coalition (CLEC), which represents 300 consumer groups, predicted a 14 percent jump.

It represents a rare bit of good news for the 45 million American families who heat their homes with natural gas and have seen their fuel bills increase an average of 125 percent during the last six years.

"In 1985 the average consumer won't see price increases of more than 5 to 10 percent," said Ed Rothschild, assistant director of the CLEC. "Some consumers won't see any increases and some may even see a decrease."

Natural gas prices, which skyrocketed from 1978 to 1983, stabilized during the last year. In the Washington area, for example, prices are only 1 percent higher than a year ago, according to Washington Gas Light spokesman Paul Young.

Factors cited as holding down price increases include the sharp drop in oil prices, a shift away from gas use by many industries, an increase in natural gas imports from Canada, the recession, weather, an oversupply of gas and cost-cutting efforts by utilities.

"The whole gas market has changed drastically. Competition and surplus has kept prices down," Rothschild said. "The assumptions of 1978 aren't valid anymore. Even the assumptions we made last summer aren't valid."

Decontrol began with the passage of the Natural Gas Policy Act in 1978, when natural gas was thought to be in short supply. The legislation, a response to the energy crisis, deregulated some natural gas prices and significantly raised the price ceilings for all gas.

The higher prices allowed under the act brought the biggest drilling binge in history and a surge of new gas. Earlier predictions of gas shortages were proven incorrect as new reserves were found in deep wells.

The biggest step in the deregulation process will occur today, when price controls on most "new" gas -- that drilled after 1977 -- end. So-called "old" gas from wells drilled earlier will remain under price control, although Energy Secretary Donald P. Hodel has vowed a new effort to seek to lift all controls in the 99th Congress, which will convene later this week.

Such efforts have failed in the past, and are not expected to succeed in 1985 as congressional leaders closely monitor the impact of decontrol.

Some analysts caution that a great deal of uncertainty remains about decontrol.

For example, the Interstate Natural Gas Association of America (INGAA), a pipeline trade group, has warned that unless pipelines are successful in renegotiating contracts with producers, wellhead gas prices will rise 9 to 12 percent beyond the rate of inflation during the year.

These contracts, negotiated in the wake of the energy crisis, gave unusually generous terms to natural gas producers. This drove up the price of natural gas, decreasing demand. At the same time, oil prices began to fall.

"There's a conflict between market realities and legal realities. Unless they are resolved, we are going to see price increases," said Catherine Abbott, INGAA vice president.

She said some companies have made progress in renegotiating "take-or-pay" clauses requiring pipelines to pay for large volumes of gas even if it cannot be sold, but other firms fear major legal problems in coming months.

Some have long-term contracts, for example, with clauses that tie the wellhead price paid by pipelines to the highest price being paid for other gas or oil in given areas.

One of these firms is Columbia Gas Transmission Corp., a principal supplier to the Washington and Baltimore areas.

Spokesmen for Washington Gas Light, which supplies fuel to two of every three residences in the Washington region, contend that this will not cause major problems in the area.

The company, spokesmen said, launched a major program in 1983 to bypass the pipeline and purchase gas directly from producers at lower prices in an effort to control cost.

Vierbuchen, WGL executive vice president, said consumers may find their bills lower this winter than last because of warmer weather. He said temperatures are about 6 percent higher this year than last.