We study the integrated WiFi/WiMAX networks, where users are equipped with dual-radio interfaces that can connect to either a WiFi or a WiMAX network. Previous research on integrated heterogeneous networks (e.g., WiFi/cellular) usually considers one network as the main and the other as the auxiliary. The performance of the integrated network is compared with the "main” network. The gain is apparently due to the additional resources from the auxiliary network. In this study, we are interested in integration gain that comes from the better utilization of the resource rather than the increase of the resource. The heterogeneity of the two networks is the fundamental reason for the integration gain. To quantify it, we design a generic framework that supports different performance objectives. We focus on the max-min throughput fairness in this work and also briefly cover the proportional fairness metric. We first prove that it is NP-hard to achieve integral max-min throughput fairness, then propose a heuristic algorithm, which provides two-approximation to the optimal fractional solution. Simulation results demonstrate significant integration gain from three sources, namely, spatial multiplexing, multinetwork diversity, and multiuser diversity. For the proportional fairness metric, we derive the formulation and propose a heuristic algorithm, which shows satisfactory performance when compared with the optimal solution.