  We have examined the physical conditions in the narrow-line region of the
well-studied Seyfert galaxy NGC 4151, using long-slit spectra obtained with the
Hubble Space Telescope/Space Telescope Imaging Spectrograph (HST/STIS). The
data were taken along a position angle of 221 degrees, centered on the optical
nucleus. We have generated photoionization models for a contiguous set of
radial zones, out to 2.3 arcsec in projected position to the southwest of the
nucleus, and 2.7 arcsec to the northeast. Given the uncertainties in the
reddening correction, the calculated line ratios successfully matched nearly
all the dereddened ratios. We find that the narrow-line region consists of
dusty atomic gas photoionized by a power-law continuum that has been modified
by transmission through a mix of low and high ionization gas, specifically UV
and X-ray absorbing components. The physical characteristics of the absorbers
resemble those observed along our line of sight to the nucleus, although the
column density of the X-ray absorber is a factor of ten less than observed. The
large inferred covering factor of the absorbing gas is in agreement with the
results of our previous study of UV absorption in Seyfert 1 galaxies. We find
evidence, specifically the suppression of L-alpha, that we are observing the
back-end of dusty ionized clouds in the region southwest of the nucleus. Since
these clouds are blueshifted, this supports the interpretation of the cloud
kinematics as being due to radial outflow from the nucleus. We find that the
narrow-line gas at each radial position is inhomogeneous, and can be modeled as
consisting of a radiation-bounded component and a more tenuous, matter-bounded
component. The density of the narrow-line gas dropswith increasing radial
distance, which confirms our earlier results.
