Grizz Chapman, the actor best known for his work on "30 Rock," died yesterday ... and now we're learning a little more about what led up to his passing.

Renee Glicker, Chapman's manager, tells TMZ ... the beloved film and TV actor has been suffering from an assortment of medical conditions in the past few years ... and was "just struggling to stay alive."

Chapman received a kidney transplant in July 2010 ... and Renee said he needed another transplant ... but given his enormous size -- he stood 7-feet tall -- finding a match again was difficult.

His exact cause of death is not clear ... but his manager suspects there are a number of contributing factors.

In addition to his work on the NBC series ... Chapman created a series of YouTube videos known as "Grizz Chroniclez" featuring him in a variety of sketches.

The actor appeared in several other films, music videos, and TV shows, including "Blue Bloods" and "The Blacklist."

Chapman is survived by his ex-wife and their two children.

He was 52.

RIP