  A bilayer system of two-dimensional electron gases in a perpendicular
magnetic field exhibits rich phenomena. At total filling factor $\nu_{tot} =
1$, as one increases the layer separation, the bilayer system goes from an
interlayer coherent exciton condensed state to an incoherent phase of, most
likely, two decoupled composite-fermion Fermi liquids. Many questions still
remain as to the nature of the transition between these two phases. Recent
experiments have demonstrated that spin plays an important role in this
transition. Assuming that there is a direct first order transition between the
spin-polarized interlayer-coherent quantum Hall state and
spin-partially-polarized composite Fermi liquid state, we calculate the phase
boundary $(d/l)_c$ as a function of parallel magnetic field, NMR/heat pulse,
temperature, and density imbalance, and compare with experimental results.
Remarkably good agreement is found between theory and various experiments.
