This paper presents a simulation framework called UNIF (Unified Numerical Integration Framework) to design numerical integration models, i.e. models mainly ruled by ordinary differential equations. We describe the object-oriented structure of UNIF, which allows to model a system as a hierarchical aggregation of subsystems interacting together, each one having a set of inputs, outputs and integration variables that are involved in a set of equations. The use and the advantages of this framework are illustrated on a full biological model, called GEMINI (Grassland Ecosystem Model with INdividual-centered Interactions), that simulates the life of populations of grassland plants competing for light and soil resources.