Yellowing on Hanging Towels

I have neglected to wash the thick, white, fluffy "company" towels in a timely fashion and now they are dingy on the outside. You know the towels I'm talking about. The ones that hang in the corner back by the shower that nobody actually ever uses. (What the heck is the point of that anyway?) I'm sure a lot of it is just everyday dust, but there is also a smoker in the house so I'm betting there's some smoke / nicotine coming into play here too.

Does anybody have any ideas how to whiten these babies back up? My mom swears by powdered dish washing detergent & dry bleach (didn't work) and an online search told me to use vinegar or alcohol (which didn't work either).

These really ARE nice (expensive) towels that I think I shall start using instead of saving for the imaginary company. But I would love to bring them back to their pristine state before doing so.

Any and all suggestions apprecated!