Heat Integration across plants is an extension of conventional Heat Integration in a single plant for further improving energy efficiency. To make good use of surplus heat, Indirect Heat Integration method using intermediate fluid loops and Direct Heat Integration method using process streams are proposed to exchange heat across plants. Up to now, the two integration methods are studied separately. This leads to an incomprehensive analysis for Heat Integration across plants, because each of the methods fit different practical situations. In this work, Combined Heat Integration method is proposed, which involves the characteristics of both direct method and indirect method. The performances for Heat Integration across plants using direct, indirect and combined methods are analyzed and compared through Composite Curves. Mathematical models are proposed to determine the optimal operation conditions for direct and indirect method. Based on a heuristic step, the optimal operation conditions for combined method can be obtained through the model for indirect method. A case study is used to illustrate the new methodology. (C) 2015 Elsevier Ltd. All rights reserved.