Utilities have been adopting repowering as an option in their plans to meet the growth in loading. In this work, repowering of older and marginally efficient steam plant by conversion to combined-cycle operation is carried out by the addition of a twin-shaft gas-turbine engine and a pressurised fluidised-bed combustor (PFBC). The low-pressure compressor rotates at variable speed to provide the PFBC with the required mass of air to satisfy various load conditions. A computer program was designed for computations of performance. Four arrangements were studied, namely the basic System 1 with a heat-exchanger interposed between the gas turbine and the PFBC, in addition to circulating the condensate of the steam plant through the gas-turbine intercooler; System 2 circulates direct cold water; System 3 cancels the heat-exchanger; System 4 cancels the heat-exchanger and raises the turbine inlet temperature to 1250 degrees C to anticipate the future performance of advanced systems. Results show that System 2 achieves marginal improvement, whereas System 3 achieves 7.3% higher power and 4% higher efficiency relative to System 1. System 4 achieves figures of 33.4 and 13.2% respectively.