In this paper, multivariant simulation calculations are performed for the following natural-gas fired combined heat-and-power plants with gas turbines: (1) a gas-steam combined heat-and-power plant with an ex traction-condensing steam turbine, and (2) a gas-steam combined heat-and-power plant with a back-pressure steam-turbine. For these systems, mathematical models of the behaviours of their basic elements, such as: the block of the gas turbine (compressor, combustion chamber and gas turbine), heat-recovery steam generator and steam-turbine cycle were developed. On the basis of elaborate mathematical models, a computer program performed multivariant simulation calculations. For each variant, the following factors were calculated for particular types of combined heat-and-power plants: energy efficiency, efficiency of electric-energy generation, cogeneration index and cogeneration factor. (C) 2003 Elsevier Science Ltd. All rights reserved.