Modern wireless systems are evolving toward a heterogeneous environment where multiple radio access technologies (RATs) coexist. Most of the RATs in current use operate independently of each other, however, there is an increasing demand for coordination among different RATs in order to support exponentially growing wireless traffic. Our goal in this paper is to design a super-MAC that integrates multiple RATs in a tightly coupled manner. To this end, we propose an architecture for super-MAC, and develop an algorithm that splits IP traffic into multiple RATs and controls each RAT to exploit multi-RAT diversity. We prove the asymptotic optimality of the super-MAC algorithm by introducing a novel reference system that enables the analysis of the multi-RAT system where there is a mixture of slotted (such as cellular) and unslotted (such as WiFi) RATs. Through extensive simulations we show that the super-MAC algorithm can enhance the quality of service including throughput, energy efficiency, and fairness.