A step in the validation of a system is to check its behavior by simulation. Simulation is also used to validate the behavior of the model of the system against test patterns. For complex systems, models are made of parts which use different modeling formalisms. The main issues in the simulation of such systems are the specification of the semantics of each modeling formalism, and of the interactions between heterogeneous parts of a model. ModHel'X relies on component based modeling and models of computation to address these problems and focuses on the computation of one possible behavior of a model. This includes simulation and code generation. ModHel'X defines a MOF meta-model for describing the structural elements of a modeling language. The semantics of modeling languages is expressed in an imperative style and addresses three aspects: control, data and time. ModHel'X is supported by a simulator of multi-formalism models.