  Double-layer quantum Hall systems possess interlayer phase coherence at
sufficiently small layer separations, even without interlayer tunneling. When
interlayer tunneling is present, application of a sufficiently strong in-plane
magnetic field $B_\parallel > B_c$ drives a commensurate-incommensurate (CI)
transition to an incommensurate soliton-lattice (SL) state. We calculate the
Hartree-Fock ground-state energy of the SL state for all values of
$B_\parallel$ within a gradient approximation, and use it to obtain the
anisotropic SL stiffness, the Kosterlitz-Thouless melting temperature for the
SL, and the SL magnetization. The in-plane differential magnetic susceptibility
diverges as $(B_\parallel - B_c)^{-1}$ when the CI transition is approached
from the SL state.
