  An implicit hydrodynamic code following the explosion of classical novae,
from the accretion phase up to the final ejection of the envelope, has been
coupled to a MonteCarlo code able to simulate their gamma-ray emission.
Carbon-oxygen (CO) and oxygen-neon (ONe) novae have been studied and their
gamma-ray spectra have been obtained, as well as the gamma-ray light curves for
the important lines (e$^{-}$--e$^{+}$ annihilation line at 511 keV, $^{7}$Be
decay-line at 478 keV and $^{22}$Na decay-line at 1275 keV). The detectability
of the emission by CGRO instruments has been analyzed. It is worth noticing
that the $\gamma$-ray signature of a CO nova is different from that of an ONe
one. In the CO case, the 478 keV line is very important, but lasts only for
$\sim$2 months. In the ONe case, the 1275 keV line is the dominant one, lasting
for $\sim$ 4 years. In both cases, the 511 keV line is the most intense line at
the beginning, but its short duration ($\sim$ 2 days) makes it very difficult
to be detected. It is shown that the negative results from the observations
made by COMPTEL up to now are consistent with the theoretical predictions.
Predictions of the future detectability by the INTEGRAL mission are also made.
