  I present a three-dimensional effective theory that describes the
high-temperature equilibrium behavior of the SU(5) theory accurately, but is
much easier to study using non-perturbative methods than the original
four-dimensional one. The effective theory is obtained by perturbatively
integrating out the super-heavy Matsubara modes as well as the heavy temporal
component of the gauge field. Regardless of the particle spectrum of the
original theory, the resulting three-dimensional theory contains only a gauge
field and an adjoint Higgs field. The phase diagram of the theory is analysed
perturbatively and it is shown to have a cellular structure in
three-dimensional parameter space.
