  The critical temperature T_c of an interacting Bose gas trapped in a general
power-law potential V(x)=\sum_i U_i|x_i|^{p_i} is calculated with the help of
variational perturbation theory. It is shown that the interaction-induced shift
in T_c fulfills the relation (T_c-T_c^0)/T_c^0= D_1(eta)a + D'(eta)a^{2 eta}+
O(a^2) with T_c^0 the critical temperature of the trapped ideal gas, a the
s-wave scattering length divided by the thermal wavelength at T_c, and
eta=1/2+\sum_i 1/p_i the potential-shape parameter. The terms D_1(eta)a and
D'(eta) a^{2 eta} describe the leading-order perturbative and nonperturbative
contributions to the critical temperature, respectively. This result
quantitatively shows how an increasingly inhomogeneous potential suppresses the
influence of critical fluctuations. The appearance of the a^{2 eta}
contribution is qualitatively explained in terms of the Ginzburg criterion.
