1. A method for determining an operating set point for a combined cycle power plant including a gas turbine, a heat recovery steam generator (HRSG), a steam turbine, and a duct firing system for the HRSG, the method comprising: using digital models to model the operation and/or the performance of the gas turbine, the steam turbine and the duct firing system; correlating the digital models using an artificial neural network, the artificial neural network interacting with the digital models; simulating the operation and/or the performance of the power plant using the digital models and the artificial neural network; correcting the simulation of the operation and/or the performance of the power plant by adjusting at least one of the digital models and/or the artificial neural network; optimizing the simulation of the operation and/or performance of the power plant by simulating the operation and/or performance of the power plant using the digital models and the artificial neural network at different operating settings of at least one of the power plant, the gas turbine, the steam turbine and the duct firing system, and selecting at least one of the operating settings as being an optimal operating setting; and setting an operating set point for the power plant based on the optimal operating setting.