  In the presence of an $\Omega$-deformation, local operators generate a chiral
algebra in the topological-holomorphic twist of a four-dimensional $\mathcal{N}
= 2$ supersymmetric field theory. We show that for a unitary $\mathcal{N} = 2$
superconformal field theory, the chiral algebra thus defined is isomorphic to
the one introduced by Beem et al. Our definition of the chiral algebra covers
nonconformal theories with insertions of suitable surface defects.
