Ideality assumes minimal interaction between the gaseous particles. For #"hydrogen chloride"#, a potent intermolecular force can operates: #"hydrogen bonding"#, in that the hydrogen is bound to a strongly electronegative chlorine to give the #""^(-delta)Cl-H^(delta+)# dipole, which can act intermolecularly.
Possibly the best metric with which to compare this is the boiling point of the gases: helium has a normal boiling point of #4*K#; hydrogen chloride boils at #188*K# #-# how is this consistent with the hydrogen bonding argument? Compare also normal the boiling point of #HF#, where hydrogen bonding should be stronger or weaker than in #HCl#?
AS hydrogen chloride also possesses many more electrons than does helium, dispersion forces will also be stronger for #HCl#.
