  Flat walls facilitate the condensation of a supersaturated vapor: Classical
theory of heterogeneous nucleation predicts that the free energy barrier
$\Delta F_{\rm het}^*$ which needs to be overcome for the formation of
sphere-cap shaped nucleation seeds is smaller than the barrier $\Delta F^*_{\rm
hom}$ for spherical droplets in the bulk by a factor $0<f(\theta)<1$, which
only depends on the contact angle $\theta$. In this letter we compute both
$\Delta F^*_{\rm hom}$ and $\Delta F^*_{\rm het}$ from Monte Carlo simulations
and test the theory for the lattice gas model (for which $\theta$ can be
readily controlled). Even though the theory is only based on macroscopic
arguments, it is shown to hold for experimentally relevant nanoscopic
nucleation seeds ($20\leq\Delta F^*_{\rm hom}/k_BT\leq 200)$ if (independently
estimated) line tension effects are considered.
