David Brown became ill in Maghaberry Prison in December 2012 and died later in hospital from a brain haemorrhage.
The Prisoner Ombudsman said staff left him unattended for five minutes in an unresponsive state and did not raise the alarm immediately.
The watchdog has concluded the response of the Prison Service was "inadequate".
The report by Prisoner Ombudsman Tom McGonigle also found that a nurse treating the inmate was not made aware that it was an emergency situation and other prisoners were not locked in their cells during the incident.
However, Mr McGonigle, said: "While some things could have been done better, a key finding of this independent investigation is that there was no possibility to achieve an alternative outcome for Mr Brown."
The Prisoner Ombudsman's office is required to investigate all deaths in custody in Northern Ireland, including deaths due to natural causes.
The report into David Brown's death said painkilling drugs were found in the 46-year-old's system during toxicology tests, but added that the drugs had been prescribed to him.
It said the medication was found at "concentrations that lay within their respective therapeutic ranges".
"This is important as there was speculation about a white powdery substance that was found around Mr Brown's nose at the time of his death," a statement from the ombudsman said.
Despite criticising prison staff for their immediate reaction to finding the prisoner unconscious in his cell, the report did not find fault with the inmate's medical management during his time in the jail.
A clinical reviewer who investigated the case "did not feel that an opportunity to achieve an earlier diagnosis existed, or that there would have been a possibility to achieve an alternative outcome for Mr Brown".
The ombudsman's report into the handling of the prison's case identified four matters that required improvement.
Two of the four areas related to record-keeping and post-incident support for staff.
The need for improvement in these two areas had already been highlighted to the prison authorities and the South-Eastern Health and Social Care Trust, which treated the inmate.
The Northern Ireland Prison Service (NIPS) has accepted the ombudman's four recommendations and said they have already been implemented.
The health trust has also accepted their recommendation, and told the ombudsman it has been reiterated to their staff and will be considered at a "Lessons Learned" forum.
Mr McGonigle has expressed sympathy to the prisoner's family.
A watchdog has criticised prison staff in Northern Ireland for their reaction when an inmate, who later died, was found "unresponsive" in his cell.