Predictive simulation systems, embedding models of real-world processes, are usually designed as stand-alone computer packages. Users wishing to work on broader problems of extended scope need access to several models. This access can be provided by integrating several packages into a decision support system offering a coherent map-based problem-oriented interface to the under-lying models. This paper proposes the FMISDS integration approach that offers a problem-oriented method for designing the integrated system. It distinguishes syntactic and semantic issues in model integration, enabling a declarative description of the models and their conceptual relationships. The integration effort is supported by a reusable toolkit that interprets the declarative descriptions. FMISDS is demonstrated in a spatial decision support system for water catchment management.