A mathematical model is proposed to calculate the characteristics of a high-temperature solid oxide fuel cell running on combustible gases obtained by thermochemical conversion of plant biomass. The calculations rely on the approximation of thermodynamic equilibrium for the reacting mixture and the polarization equations recommended in the literature. The proposed mathematical model is used to assess the influence of the produced gas composition (obtained under different conditions) on fuel cell performance. The study has identified the gasification conditions which allow producing syngas whose electrochemical conversion is the most efficient.