The management of municipal solid waste (MSW) and the current status of world energy resources crisis are important problems. Gasification is a kind of waste-to- energy conversion scheme that offers the most attractive solution to both waste disposal and energy problems. In this study, the thermodynamic equilibrium model based on equilibrium constant for predicting the composition of producer gas in a downdraft waste gasifier was developed. To enhance the performance of the model, further modification was made by multiplying the equilibrium constants with coefficients. The modified model was validated with the data reported by different researchers. MSW in Thailand was then used to simulate and to study the effects of moisture content (MC) of the waste on the gasifier's performance. The results showed that the mole fraction of H-2 gradually increases; CO decreases; CH4, which has a very low percentage in the producer gas increases; N-2 slightly decreases; and CO2 increases with increasing MC. The reaction temperature, the calorific value, and the second law efficiency, decrease when MC increases. (c) 2007 Elsevier Ltd. All rights reserved.