Following Alexei Zamolodchikov, we consider the conformal blocks in the theories with extended conformal W-symmetry for the integer Virasoro central charges. We show that these blocks for the generalized twist fields on sphere can be computed exactly in terms of the free field theory on the covering Riemann surface, even for a non-abelian monodromy group.  The result is elegantly formulated using the technique from gauge theory/CFT correspondence.