A precise prediction of the power generation capacity of each power plant in an electric grid is important for managing the grid stably by preventing mismatch between the electric demand and power production. This prediction also helps power plant owners maintain their facilities because it allows effective performance monitoring. This paper proposes a unique analysis tool for predicting the near term power generation capacity of cogeneration combined cycle power plants using correction curves and real operating data. The plant operation was simulated using the well-simulated design performance and an off-design calculation model consisting of performance correction curves and thermodynamic modeling. Real operation data of several seven-day periods were simulated using the developed tool. The data of the first five days was used as a tool set up leading to a calculation of the power correction factor. The obtained power correction factor reflected the performance degradation quite well. The data of the last two days was used for tool validation. The discrepancy between the predicted and measured power outputs were less than 2%. The proposed method can be used for both performance monitoring and a prediction of the power generation capacity of gas turbine-based power plants. (C) 2017 Elsevier Ltd. All rights reserved.