The present work is focused on a Computational Fluid Dynamics (CFD) model of a real-scale waste-toenergy plant, accounting for the coupling between thermo-chemical conversion of solid Refuse-Derived Fuel (RDF) and gaseous combustion of released syngas. The first process is simulated through an in-house two-zone zero-dimensional model, consisting of solid RDF drying and its conversion into syngas, while the turbulent gaseous combustion taking place in the freeboard is simulated by a 3D CFD model developed within the Ansys FLUENT environment. This approach, validated in a previous work, is here more deeply analysed, performing a parametric analysis of the two-zones thermo-chemical conversion model to evaluate the effects of the extension of the drying zone on the whole simulation process and to quantify the limits related to this hypothesis. The coupled model predictive capability is tested in different conditions, varying the amount of drying and gasifying agent mass flow rates, to verify the model physical consistency and analyze the effects of the main governing parameters on biomass conversion. The influence of the radiative thermal power on the RDF moisture evaporation rate is assessed, while the gaseous combustion is described through a more accurate chemical kinetics mechanism, determining a low increase in the computational cost.