Highly efficient thermal conductors, such as the heat pipes, can serve as the main paths for heat flux in heat transfer systems. By optimizing positions of these conductors in a heat transfer system, significantly higher efficiency can be achieved. This paper studies the optimization of the integrated heat-transfer systems consisting of conventional solid materials and embedded thermal conductors using the stiffness spreading method, which is a novel optimization model recently proposed for the layout optimization of truss and multi-component structures. A numerical model for the layout optimization of such multi-component systems is proposed to optimize the positions of thermal conductors as well as the distribution of conductive materials. The sensitivities of the objective function with respect to design variables are derived analytically, and thus the formulated optimization problem can be solved efficiently using the method of moving asymptotes. Numerical examples are presented to demonstrate the effectiveness and potential applications of the proposed model. (C) 2017 Elsevier Ltd. All rights reserved.