Underground coal gasification is a candidate process for converting the world's coal resources into energy, liquid fuels, and chemicals. In this article, a one-dimensional model of a reacting coal block called COALGAS is used to investigate the effects of operating conditions and coal properties on the local rate of cavity growth and energy effectiveness in underground coal gasification. The investigation has revealed that the cavity growth rate is most sensitive to the operating temperature, water influx, and gas pressure. The coal properties that most affect the cavity growth rate are the thermomechanical spalling behavior, the behavior of the ash, and the amount of fixed carbon in the coal. Many trends observed in the field trials are reproduced by the model simulations, and predicted cavity growth rates for six field trials are comparable to those observed.