This paper describes an environment for performance-oriented design of portable parallel software. The environment consists of a graphical design tool based on the PVM communication library for building parallel algorithms, a state-of-the-art simulation engine and a visualization tool for animation of program execution and visualization of platform and network performance measures and statistics. The toolset is used to model a virtual machine composed of a cluster of workstations interconnected by a local area network. The simulation model used is modular and its components are interchangeable which allows easy re-configuration of the platform. The model is validated using experiments on the COMMSI benchmark from the Parkbench suite, and a standard image processing algorithm with maximum errors of 1% and 10% respectively.