We present ModHel'X, an approach to multi-formalism modeling which addresses two important issues in this field: (a) providing support for the specification of the semantics of a modeling formalism, and (b) allowing the specification of the interactions between parts of a model described using different modeling formalisms. ModHel'X is based on the concept of Model of Computation and focuses on the execution of models, considered as the computation of one possible behavior of the model. The structural elements of a modeling language are described by specializing the meta-model of ModHel'X whereas its semantics, i.e. the corresponding model of computation, is described by specializing the predefined stages of a generic execution model. Using the same mechanisms, designers can specify the semantic adaptation that is suitable at each interface between heterogeneous parts of their model. Finally, ModHel'X comes with an execution engine which is able to interpret heterogeneous models for simulation.