  We investigate the influence of environment on the cold-gas properties of
galaxies at z=0 within the TNG100 cosmological, magnetohydrodynamic simulation,
part of the IllustrisTNG suite. We extend previous post-processing methods for
breaking gas cells into their atomic and molecular phases, and build detailed
mocks to comprehensively compare to the latest surveys of atomic hydrogen (HI)
in nearby galaxies, namely ALFALFA and xGASS. We use TNG100 to explore the HI
content, star formation activity, and angular momentum of satellite galaxies,
each as a function of environment, and find that satellites are typically a
factor of ~3 poorer in HI than centrals of the same stellar mass, with the
exact offset depending sensitively on parent halo mass. Due to the large
physical scales on which HI measurements are made (~45--245 kpc), contributions
from gas not bound to the galaxy of interest but in the same line of sight
crucially lead to larger HI mass measurements in the mocks in many cases,
ultimately aligning with observations. This effect is mass-dependent and
naturally greater for satellites than centrals, as satellites are never
isolated by definition. We also show that HI stripping in TNG100 satellites is
closely accompanied by quenching, in tension with observational data that
instead favour that HI is preferentially stripped before star formation is
reduced.
