In part A of this study, investigations into the capability of different process simulation codes to predict the real behaviour of a combined cycle power plant during part loads and off-design operation are carried out. The combined cycle power plant with a sub-critical three-pressure-stage heat recovery steam generator is built using the process simulation software tools Aspen Plus Dynamics and Apros. The generated numerical models are validated against the measurements at different steady-state operation points, namely 100%, 80% and 60% loads. The numerical results obtained display a very good agreement towards the given data with a maximal relative error of about 5% for the pressure and less than 1% for the temperature and the mass flow rate. Furthermore, the validated models are also evaluated against the dynamic measurements during a transient operation for a time period of 400 min. Apros and Aspen Plus Dynamics models predict qualitatively the right developments of the plant parameters. However, the simulated results deviate quantitatively from the experimental data with a maximum relative error of about 10%. It can be concluded that the dynamic simulation models can represent accurately the real combined cycle power plant at different steady-state loads, while they can capture the plant behaviour during the off-design operation with less accuracy. (C) 2015 Elsevier Ltd. All rights reserved.