This work aims to develop mathematical models to simulate a single shaft gas turbine based cogeneration plant with variable geometry compressor. At off-design the variable vanes are re-staggered to improve the cogeneration performance. Two modes of operation are identified with the first mode being for part load of less than 50 running to meet the part load demand. This is achieved by controlling the fuel flow and air bleeding at the downstream of the compressor to avoid surge formation. The second mode of operation is for part load greater than 50. It is running to meet both the part load demand and the exhaust gas temperature set value by simultaneously regulating the fuel flow and the variable vanes opening. To accommodate change of compressor parameters during variable vanes re-stagger correction coefficients are introduced. A behavior of a 4.2 MW power generating cogeneration plant is simulated. The effect of variation of power and ambient temperature on cogeneration parameters like fuel consumption, temperature, pressure, variable vanes opening, efficiency and steam generated is studied. Comparison between the field data and the simulation results is in good agreement. To support the calculations required for off-design analysis, a computer program is developed in MatLAB environment. © 2010 Elsevier Ltd. All rights reserved.