  The charged-Higgs boson contributions to the Wilson coefficients C_7 and C_8,
relevant for the decay B -> X_s gamma, are discussed in supersymmetric models
at large tan beta. These contributions receive two-loop O(alpha_s tan beta)
corrections by squark-gluino subloops, which are possibly large and
nondecoupling in the limit of heavy superpartners. In previous studies, the
relevant two-loop Feynman integrals were approximated by using an effective
two-Higgs-doublet lagrangian. However, this approximation is theoretically
justified only when the typical supersymmetric scale M_SUSY is sufficiently
larger than the electroweak scale m_weak \sim (m_W,m_t) and the mass of the
charged-Higgs boson m_{H^\pm}. Here we evaluate these two-loop integrals
exactly and compare the results with the existing, approximated ones. We then
examine the validity of this approximation beyond the region where it has been
derived, i.e. for m_H \gtap M_SUSY and/or M_SUSY \sim m_weak.
