  To account for the non-thermal emission from the classical nova V2491 Cygni,
we perform a series of numerical calculations of radiative transfer of
\gamma-ray photons from the radioactive isotope $^{22}$Na in the matter ejected
from a white dwarf. Using a simple wind model for the dynamical evolution of
the ejecta and a monte-carlo code, we calculate radiative transfer of the
\gamma-ray photons in the ejecta. Repeated scattering of the \gamma-ray photons
by electrons in the ejecta, i.e., Compton degradation, results in an extremely
flat spectrum in the hard X-ray range, which successfully reproduces the
observed spectrum of the X-ray emission from V2491 Cygni. The amount of the
isotope $^{22}$Na synthesized in the ejecta is required to be 3\times 10^{-5}
M_\odot to account for the flux of the hard X-ray emission. Our model indicates
that the ejecta become transparent to the \gamma-ray photons within several
tens days. Using the results, we briefly discuss the detectability of the
\gamma-ray line emission by the {\it INTEGRAL} gamma-ray observatory and the
{\it Fermi} Gamma-ray Space Telescope.
