The paper deals with the implementation in a continuous simulation language of the dynamic model of an Integrated Generation System (IGS) named "Combined Multiple Renewable Energy Sources System Simulator" (CMRESSS). The software package is a useful tool to evaluate the IGS electric transient behavior during the planning stage. Dynamic transients in normal and faulted conditions are simulated exploiting the main features of the package which are flexibility and Graphic User Interface (GUI) for easy handling of input data and events scheduling, Simulation results are discussed and validated to demonstrate the validity of the prososed approach. The results confirm the consistence of the simulation package and the benefits achievable in preliminary design by comparative analysis of different power plants.