A mathematical model which was developed to predict steady state performance of a biomass downdraft gasifier/spark ignition engine power system is described. A mathematical model of the integrated system consists of two parts: the fixed bed downdraft gasifier and spark ignition internal combustion engine models. For calculations the gasifier is split into three zones, namely drying e pyrolysis, oxidation and reduction sections. The gasifier’s mathematical model consists of three separate sub-models, each describing the processes in the corresponding zone. The process taking place in the reduction zone has been described using chemical kinetic principles in order to avoid introduction of assumptions related to achievement of the thermo-chemical equilibrium state during gasifier’s operation. The model is capable to accurately predict molar concentrations of different species in syngas (CO2, CO, H2O, H2, CH4 and N2)and the temperature profile in the gasifier along its height. This information then can be used for sizing the reactor and material selection. The engine’s model is based on the fueleair thermodynamic cycle for spark ignition engines and such model takes into account the composition of syngas used as fuel. The engine’s model also takes into account effects of heat losses in the cycle through the walls of the cylinders and due to the gas blow by. Finally, the influence of dissociation processes during the combustion and the residual gases remaining in the cylinders at the beginning of the compression stroke is accounted for computations of the engine’s performance. The numerical results obtained using the proposed model are in a good agreement with data produced with the use of other theoretical models and experimental data published in open literature and with experimental data obtained in these investigations. The proposed model is applicable for modelling integrated downdraft gasifier/engine biomass energy systems and can be used for more accurate adjustment of design parameters of the gasifier and the engine in order to provide the higher overall efficiency of the system.