The utilization of coal to produce a syngas via gasification processes is becoming a sustainability option because of the availability and the economic relevance of this fossil source in the present world energy scenario. Reserves of coal are abundant and more geographically spread over the world than crude oil and natural gas. This paper focuses on sustainability of the process of coal gasification: where the synthesis gas may subsequently be used for the production of electricity, fuels and chemicals. The coal gasifier unit is one of the least efficient step in the whole coal gasification process and sustainability analysis of the coal gasifier alone can substantially contribute to the efficiency improvement of this process. In order to evaluate sustainability of the coal gasification process energy efficiency, exergy based efficiency, exergy destruction factor, environmental destruction factor, sustainability index and improvement potential are proposed in this paper. (C) 2011 Elsevier Ltd. All rights reserved.