  We present numerical simulations of radiative transfer in the spiral galaxy
NGC 6946. The interstellar medium is represented as a two phases medium, with
molecular clouds and a smooth diffuse phase. The molecular gas distribution is
calculated from the distribution of an ensemble of molecular clouds evolving in
the gravitational potential of NGC 6946. We simulate star formation by creating
OB associations in molecular clouds. The transfer of UV radiation is calculated
in the clumpy interstellar medium, to determine the local UV illumination of
molecular clouds. We compute the emergent intensity in the UV continuum, in the
Halpha and C+ lines as well as in the continuum at far infrared wavelengths,
60, 100 & 200 microns. It is possible to obtain a consistent picture of this
galaxy with a global star formation rate of 4 Mo/yr occuring mostly in the
spiral arms. The UV opacity of the model spiral galaxy disk, observed face-on,
is 0.8 at 100 nm.
  For radii less than 4 kpc, the C+ line is mostly produced in
photodissociation regions at the surfaces of molecular clouds. The C+ emission
from diffuse atomic gas accounts for about 20% of the total. It becomes
significant at large distance from the nucleus (r > 4 kpc). Molecular clouds
and diffuse atomic gas have almost equal contributions to the total far
infrared emission from 60 to 200 microns. As a whole, 72% of the 60 - 200
microns FIR emission can be attributed to dust grains heated by the UV
radiation of massive stars and 28% by the radiation field of the old stellar
population.
