By incorporating reaction kinetics and reactor hydrodynamics, a steady-state two-phase one-dimensional reactor model for biomass steam gasification in the bubbling-fluidized bed gasifier of a dual fluidized bed reactor is developed. The generic two-step kinetic model adopted for biomass pyrolysis allows for prediction of tar generation and cracking, as well as predicting pyrolysis product yield and composition based on CHO elemental balances. This model is capable of predicting species concentrations, solids holdup, temperature, pressure, and superficial gas velocity profiles along the gasifier. By performing mass and energy balances over the two interconnected fluidized beds, key operating parameters such as solids circulation rate and additional fuel required for stable operation of the process are approximated. This predictive reactor model, which will provide a useful tool for designing, evaluating, and improving a dual fluidized bed gasifier, is compared with experimental data from the pilot dual fluidized bed gasification unit at the University of British Columbia.