Plasma gasification melting (PGM) is a promising gasification technology aiming at providing sustainable disposal for various wastes. In this work, an Euler-Euler multiphase model was developed to study the characteristics of air and steam gasification of municipal solid waste in a PGM reactor. The model is validated by measurement data from a demonstration PGM reactor. With this model, three groups of simulations were performed to study the influences of operating conditions. It is confirmed that injection of high temperature steam is important for increasing the cold gas efficiency and syngas lower-heating-value. The effect of steam injection is affected by steam feeding rate, air feeding rate and plasma power. Based on the simulated results, an optimal condition is suggested for air and steam gasification in the PGM reactor. (C) 2012 Elsevier B.V. All rights reserved.