Oxygen-blown biomass integrated gasification combined cycle (IGCC) plants are one of the most promising options for clean energy generation with CO2 abatement potential. However, the integrated nature of IGCC leads to difficult design problems. In this study, we present an advanced simulation environment for the preliminary design and retrofit of IGCC plants. We describe the modelling approach, the model validation strategy and the plant behaviour, as determined by sensitivity analyses. The simulation environment uses Pareto curves to examine various co-gasification and co-production case studies in terms of technical, economic and environmental performance. It serves as a decision support tool in the design stage, which can be used to explore ways to improve plant performance and to analyse the influence of raw materials and the unit’s operational parameters. The test and validation results are discussed.