The use of simulation modelling in science and engineering is today well established. Three methods exist that allow to build complex simulation models in many different contexts: Discrete Event Simulation, System Dynamics, and Agent Based Simulation. Despite the interesting results obtained, what has really limited the parallel and combined application of the three currently used paradigms was the lack of integrated simulation tools. This paper presents the outcomes of a complex plant simulation that has been carried out by means of the parallel and combined application of Discrete Event Simulation and Agent Based Simulation, while using a well-structured modular approach. This presents the advantage of simplifying the building, the debugging and the validation of the whole simulation model, while, at the same time, it offers the unquestionable benefit of allowing a widespread reusability of the simulation code.