The wide scale conversion of apartments to condominium ownership in this Rocky Mountain city - fueled by Denver's growing status as the energy capital of the nation - is displacing elderly and low-income renters in increasing numbers.

Realtors estimate that 6,000 units have been converted since Jan. 1, and they say the trend will continue. Denver's apartment vacancy rate is below 2 percent, construction of apartment buildings is at a standstill, and rental rates are increasing, sometimes as much as 40 percent in a month.

Meanwhile, in Denver's suburbs, workers continue to build single-family houses priced at up to $150,000. Downtown the evidence of boomtown is everywhere: High-rises for major oil firms and energy-related companies are erected as fast as technology allows.

The state's population has increased by 53,000 persons in the last year, to nearly 2.7 million, and three out of five newcomers have settled in the Denver area.

It is no longer profitable to build and operate apartment houses, given soaring land costs, interest and tax reates according to real estate spokesmen. Selling renovated apartments as condominiums provides a quicker, better return for investors, they say.

Foreign investors have driven up prices for apartment buildings by paying vastly inflated amounts, said apartment management executive R. Don Larrance. The only way to get a reasonable return, he said, is to convert.

Canadians, Arabs, Californins and investors from Salt Lake city are all interested in Denver.

"Denver is hot because it will shortly become the energy capital of the world," said, Keith F. Schefer, a real estate broker. "We have more coal than the foreigners have oil. We are the center for solar energy research. We have uranium. We've got oil shale. We've got, to a certain extent, oil and gas."

International firms such as Mobil, Sinclair and Atlantic-Richfield are expanding or bringing regional offices to Denver. Banks and mortgage companies are coming too, said Henry Mosgrave, executive vice president of Aristec Crop., a real estate investment and management firm.

Part of the attraction is Denver's location, which makes it a convenient transportation center. The city's climate and recreational offerings also add to its desirability.

But Denver's boomtown success is a doubled-edged sword. While foreign and local investors trade apartment buildings and make the decision to convert, residents of those buildings are affected in ways that can cause hardship and lifestyle changes.

Marie Church is 76 and a widow. For 10 years she has lived in a three-story apartment building at 1175 Emerson St. in the Capitol Hill district. This week workmen were pounding on the roof of her apratment while she looked, disoriented, at the gold loveseat and Elizabethan sofa that will be sold when she vacates the apartment to later this month. Her apartment will be enovated and sold as a condominium.

"I never thought a thing like this could even happen," she said. I can hardly stand it for the racket."

She was given a month's notice about the conversion, she said, and was asked if she would like to buy the five-room unit for $44,000. She plans to move.

Everett Lee is 64. The rent for his small, Capitol Hill studio increased from $155 to $200 this month, and he was notified that it will be converted. Lee, who has fixed income, has begun the search for another place.

Many elderly persons complain that even if they could afford condominiums, they would not be interested; they feel they are too old to acquire property.

Moving from one residence to another is traumatic for many older persons, said Sandy Goldhaber, a social worker. "When they move, they get lost. They have problems with recent memory. They don't know where the store is. They may look for the manager they've always known."

There is a six-month waiting list for public housing Goldhaber said. Increasingly, social workers are placing dispossessed applicants in more troubled neighborhoods away from the city's center. People are not being placed in suburgs, however, which have little or no low-income housing.