In combined heat and power systems, varying mass flow can better make use of the heating system inertia to increase the flexibility of electric power systems. This is challenging, however, due to integer variables and bilinear constraints in existing optimal dispatch models. In this paper, we incorporate an improved heat pipeline model to eliminate complexity from integer variables without compromise on accuracy. Subsequently, the resulting optimal dispatch model with bilinear constraints is solved by the proposed modified Generalized Benders Decomposition method, which decomposes the optimal dispatch model into a convex sub-problem with the fixed mass flow and a simple upper-level problem searching for the optimal mass flow. Comparisons with existing benchmarks show that the proposed method can achieve lower operation costs with outstanding computational efficiency.