  The inelastic cross sections and rate coefficients for the rotational
excitation of methanimine (CH$_2$NH) by cold H$_2$ have been determined quantum
mechanically based on a new highly correlated five-dimensional potential energy
surface. This surface was fitted to more than 60 000 ab initio points with a
root mean square error of $\sim$ 1-2 cm$^{-1}$ in the region of the potential
well whose depth is -374.0 cm$^{-1}$. The rotationally inelastic rate
coefficients have been combined with spectroscopic data to generate the
non-equilibrium spectrum of CH$_2$NH towards the interstellar molecular cloud
Sgr~B2(N). The transition $1_{10}\to 1_{11}$ at 5.29 GHz is found to be
inverted and the predicted weak maser emission spectrum is in excellent
agreement with new observations performed with the 100-meter Green Bank
Telescope.
