A case study of total process energy-integration in a commercial ammonia plant was performed using a modified pinch analysis. Several tools, including Grid Diagram, Composite Curve (CC), Grand Composite Curve (GCC), Balanced Grand Composite Curve (BGCC) and Splitting Grand Composite Curve (SGCC), were adapted in the diagnosis of process energy-utilizing. It is shown that the utility loads calculated by operating pinch calculation (OPC) were very close to those in the existing process, and that the method could be used to describe the heat-flow profile of the plant. Some inefficient energy use in the plant was observed in the calculation. The design pinch calculation (DPC) revealed that 1150 kg/h of fuel gas (natural gas) and 1322 t/h of cooling water could be saved through optimizing the temperature difference contribution value (DeltaT(c)) of heat transfer in each stream for an existing ammonia plant. And a proposal for energy saving is presented. The energy-integration technology showed great promise in the retrofit of large-scale complex process. The successful application of this technology in more than 10 Chinese plants has generated a profit of about 80 million RMB per year. (C) 2003 Elsevier Ltd. All rights reserved.