Pneumonia Played Factor in Juan Gabriel's Death

Juan Gabriel's history of pneumonia was among several significant factors that contributed to his death ... this according to the death certificate obtained by TMZ.

The document confirms the iconic Mexican singer's death was caused by arteriosclerotic cardiovascular disease ... which prevents blood flow to the heart.

TMZ broke the story ... Juan's heart problems led to his sudden death nearly 2 weeks ago. He also battled diabetes. Along with pneumonia, the death certificate lists hypertension as a factor.

You'll recall ... pneumonia forced the singer to cancel numerous shows late in his career.

He was 66.