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Recent advances in wireless technology have made it increasingly feasible to equip wireless nodes with multiple radios, thereby allowing each radio to exploit channel diversity in the form of orthogonal, non-overlapping transmission spectrums. Multi-channel operation mitigates interference, but at the same time raises new challenges for network optimization, in terms of judicious channel assignment for efficient bandwidth utilization. While previous research mostly studies optimizing channel assignment for unicast, we focus instead on multicast, which is an efficient mechanism for one-to-many data dissemination. We derive a model for optimal multicast in multi-channel multi-radio wireless networks under the assumption that channel assignment is static. Our model employs network coding as the multicast mechanism of choice, and exploits the broadcast nature of omnidirectional antennas for efficient bandwidth utilization. Based on the model derived, we formulate optimal multicast as a linear integer program. Two accompanying solutions are proposed: a greedy channel assignment scheme and an improved iterative scheme inspired by primal-dual algorithm design. The effectiveness of the two schemes are empirically examined through simulation studies, and are compared to results obtained from solving the integer program as well as its linear programming relaxation. Finally, we present an alternate model for optimal multicast under the assumption that transmission frequencies are not fixed divisions of the usable spectrum.