The approximation of a heavy gaseous moderator is used as an illustrative example for the discussion of calculational methods in the thermal and epithermal energy regions. The energy distribution and migration area of neutrons in a infinite homogeneous medium are calculated numerically for 1/ν absorption. Semianalytic expressions are obtained for the case of weak absorption. Finally, an expression for the slowing-down density is given and its significance for the modification of age-diffusion theory to include the thermal motion of the moderator is discussed.