This paper presents a static and dynamic simulation model of a supercritical once-through heat recovery steam generator (SC HRSG) and its application to investigate the load changes and start-up processes for next generation high efficiency combined cycles. The scale and complexity of the modern combined cycle power plants based on high pressure and temperature steam requirements are increasing. To design such a system, it is necessary to generate detailed computer models. This study describes the upgrading of a validated sub-critical heat recovery steam generator model to a supercritical once-through heat recovery steam generator up to 250 bars in the high pressure circuit. The SC HRSG model is built using commercial simulation software named Advanced Process Simulation Software (APROS). The model includes advanced control circuits to have a high level of accuracy, especially during start-up. The comparison between the simulation results and the designed data in steady state and different load changes are documented. The obtained results demonstrate that the simulation is very reliable to predict the start-up procedure for the SC HRSG. Through a comparative study, the differences in the thermal efficiency and the steam parameters between super/sub-critical heat recovery steam generators during warm start-up process are assessed. (C) 2008 Elsevier Ltd. All rights reserved.