  The results of semi-classical calculations of rate coefficients of
$(n-n')$-mixing processes due to collisions of Rydberg atoms $He^{*}(n)$ with
$He(1s^2)$ atoms are presented. It is assumed that these processes are caused
by the resonant energy exchange within the electron component of $He^{*}(n)+He$
collision system. The method is realized through the numerical simulation of
the $(n-n')$-mixing processes, and is applied for calculations of the
corresponding rate coefficients. The calculations are performed for the
principal quantum numbers $n,\mbox{} n'$ in ranges $4 \le n < n' \le 10$, and
the atom and electron temperatures, $T_a, T_e$, in domains $5000K \le T_{a} \le
T_{e} \le 20000K$. It is shown that the $(n-n')$-mixing processes can
significantly influence the populations of Rydberg atoms in non-equilibrium
weakly ionized helium plasmas with ionization degree $\sim 10^{-4}$. Therefore,
these processes have to be included in the appropriate models of such plasmas.
