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Thermoelectric modules are a promising approach to energy harvesting and efficient cooling. In addition to the longitudinal Seebeck effect, recently transverse devices utilizing the anomalous Nernst effect (ANE) have attracted interest. For high conversion efficiency, it is required that the material should have a large ANE thermoelectric power and low electrical resistance, the product of which is the ANE conductivity. ANE is usually explained in terms of intrinsic contributions from Berry curvature. Our observations suggest that extrinsic contributions also matter. Studying single-crystal MnBi, we find a very high ANE thermopower (~10 $μ$V/K) under 0.6 T at 80 K, and a transverse thermoelectric conductivity of over 40 A/Km. With insight from theoretical calculations, we attribute this large ANE predominantly to a new advective magnon contribution arising from magnon-electron spin-angular momentum transfer. We propose that introducing large spin-orbit coupling into ferromagnetic materials may enhance the ANE through the extrinsic contribution of magnons.