Hope you're not getting sick of these! It's a dense book, I am trying to learn and remember everything I can about it.
Anyway, this is a narrator obsessed with clarity, grammatical and otherwise. People find Infinite Jest a hard read and slow going, but I'm convinced it's because of the physical design of the physical book, which was somewhere between "not very good" and "medieval torture device" depending on which edition you got. (Only the hardback was even close to usable, as far as I can tell.) On a sentence by sentence level it's an unusually easy read, really, if you're a regular reader: it sacrifices beauty and snappiness for clarity on a regular basis. (Something I need to remember myself.)
But of course in every voice there are opportunities for new and unexpected beauties, and so here's one of them: when a sentence gets away from the narrator a little, to the point where the subject gets a little buried and easily forgotten, and so the next sentence is a fragment designed to clarify the previous sentence's subject. Like so:
"About for months into his Ennet House residency, the agonizing desire to ingest synthetic narcotics had beeen mysteriously magically removed from Don Gately, just like the House Staff and the Crocodiles at the White Flag Group had said it would if he pounded out the nightly meetings and stayed minimally open and willing to persistently ask some extremely vague Higher Power to remove it. The desire."
So, right: the sentence is long, as many of the sentences in the book are long. But it's got discrete units and to the extent that one element carries through -- the subject of "the desire to ingest synthetic narcotics," which is the antecedent to both "it"s in the sentence -- Wallace makes sure to help us remember what "it" is, going so far as to dedicate a sentence fragment to that purpose. But this isn't just clarification, which would be enough; it's beautiful. The way "the desire to ingest synthetic narcotics" is reduced to "The desire." The way the fragment makes those words seem for a moment like the universe, or a model thereof. It's gorgeous stuff. Wallace does it several times. Something to keep in mind.