Increasing deployment of combined heat and power plants intensifies interactions between renewable-based electric power systems and district heating systems. Combined heat and power dispatch is thus critical for energy-efficient coordination of integrated electricity and heat systems under high penetration of renewable energy. However, the complicated nature of district heat flows introduces significant nonlinearities and precludes the practical implementation. In this paper, a combined heat and power dispatch method based on a simplified district heat flow model is proposed to enhance the computational efficiency with desirable accuracy for better operational decisions. Several novel modifications are incorporated to reduce the model size and relieve the model nonlinearity through analyzing operational principles of district heating networks. An iterative solution algorithm is developed accordingly based on sequential linear programming to boost computational efficiency and guarantee fast convergence. The proposed method can describe temperature dynamics of district heating systems with respect to transmission delays and heat losses without assumptions on constant mass flow rates or constant supply temperature. Case studies based on a test system validate the effectiveness of the proposed method.