  We study the 6d N=(0,2) superconformal field theory, which describes multiple
M5-branes, on the product space S^2 x M_4, and suggest a correspondence between
a 2d N=(0,2) half-twisted gauge theory on S^2 and a topological sigma-model on
the four-manifold M_4. To set up this correspondence, we determine in this
paper the dimensional reduction of the 6d N=(0,2) theory on a two-sphere and
derive that the four-dimensional theory is a sigma-model into the moduli space
of solutions to Nahm's equations, or equivalently the moduli space of
k-centered SU(2) monopoles, where k is the number of M5-branes. We proceed in
three steps: we reduce the 6d abelian theory to a 5d Super-Yang-Mills theory on
I x M_4, with I an interval, then non-abelianize the 5d theory and finally
reduce this to 4d. In the special case, when M_4 is a Hyper-Kahler manifold, we
show that the dimensional reduction gives rise to a topological sigma-model
based on tri-holomorphic maps. Deriving the theory on a general M_4 requires
knowledge of the metric of the target space. For k=2 the target space is the
Atiyah-Hitchin manifold and we twist the theory to obtain a topological
sigma-model, which has both scalar fields and self-dual two-forms.
