This paper reports on our research into supporting collaborative distributed applications on the Grid. Our case study application, a Virtual Reactor problem solving environment, was built for simulation of industrially important technology of plasma chemical deposition. It incorporates a number of components distributed among different organizations and requires bothhigh-performance and high-throughput computing, as well as interactivity and advanced visualization. The application has been ported to the Grid environment and tested on a highly heterogeneous infrastructure of the RidGrid testbed. We describe the Virtual Reactor architecture and approaches used to integrate distributed components of the application on the Grid, emphasizing the role of the Migrating Desktop Grid portal and tools that bridge the gap between Grid resources and end-user applications. We outline the main features and functionalities of our RidGrid testbed built within a larger European CrossGrid infrastructure, and describe novel solutions and technical approaches that facilitate the usage of Grid resources and optimize load balancing for parallel applications on heterogeneous resources.