Thermo-catalytic cracking processes such as pyrolysis and gasification have great potential to convert carbon based solid waste to useful chemicals and energy. Although much attention has been devoted to gasification in recent years, there are still a number of challenges to full commercialisation of solid waste gasification. This work deals with two stage pyrolysis/gasification systems with great potential to produce tar free, high hydrogen content syngas from different types of heterogeneous solid waste. Advanced Aspen Plus simulation was employed in the modelling and optimization of a two-stage pyrolysis/gasification of lignocellulosic waste biomass (LWB) and refuse-derived fuel (RDF). Data required for process simulation, such as raw material elemental composition, heating value and particle size distribution, were determined by experimental measurements. Raw materials used in the process (RDF and LWB) were characterised by thermogravimetric (TGA) analysis, differential scanning calorimetry (DSC), elemental analysis and bomb calorimetry. The simulator process conditions were set based on experimental observations in laboratory scale pyrolysis and gasification experiments. Simulation of different scenarios and optimization of process parameters are presented. Based on the simulation results, under optimal conditions from each kg of RDF, 2.1 kg of gas with the heating value of 6.94 MJ/Nm3, and from each kg of LWB, 1.78 kg of gas with the heating value of 7.27 MJ/Nm3 can be obtained.