Systems of thermal diagnostic of an operation require computational tools, including mathematical models. These models should have a simple structure and short computing time. Therefore, often analytical-empirical models are used which encompass both mass and energy balances as well as additional empirical functions whose coefficients are estimated on the basis of measurement results. As a result, changing technical conditions of modelled machines are taken into account. This paper presents the simulation model of an existing gas-steam combined heat and power plant, which has a modular design and is divided into three partial models of: the gas turbine unit with rated power of 125.4 MW, the double-pressure heat recovery steam generator and the steam-water cycle with steam turbine rated 65 MW. The simulation models allow to calculate non-measured operating parameters and energy assessment indicators. They also have the capability of adapting to the changing technical conditions of the modelled machines. The developed models were validated by the use of measurements. Model predictive quality was verified with the determination factor and root mean square error. The models were also used to simulate the behavior of the analyzed gas-steam CHP plant under different operating conditions. Exemplary calculations have been presented. (C) 2018 Elsevier Ltd. All rights reserved.