Experiments carried out in an indirectly heated biomass gasifier successfully demonstrated that throughput and gas heating value could be increased by using a latent heat ballast. A mathematical model describing heat transfer and chemical reaction in the system was developed. The heat transfer submodel is accurate to within 13 K for both ballasted and unballasted scenarios. However, the chemical submodel, based on thermodynamic equilibrium calculations, proved inadequate and underpredicted cooling times during the pyrolysis phase of the process by about 50%. Improvement was made by substituting the measured product gas composition for the calculated gas composition.