A method is described for significantly improving the predictive capability of equilibrium-based calculation tools for the estimation of fuel gas composition in high-temperature biomass gasification processes. It is based on acknowledgment of the fact that the overall thermochemical conversion occurs in two stages: fast biomass-particle devolatilization, followed by the much slower, and hence often nonequilibrium, conversion of methane and char. Inputs to the equilibrium calculation routine, modified in relation to this latter phenomenon, are proposed that can be readily implemented in commercial flow sheet calculation routines for chemical reactors. The method provides estimates for the yields of specific compounds, such as hydrogen and carbon monoxide, and is shown to predict well the performance trends relating to changes in operating conditions. None of the available kinetic models is able to offer comparable predictive capability. Nor too do thermodynamic equilibrium model methods that fail to take into account the two-stage nature of the gasification process and, as a result give rise to substantial deviations from available experimental data: underprediction of both the methane content in the product gas and the unconverted carbon in the solid phase; negative findings similar to these have been reported in analogous literature studies of coal gasification.