In this important poem by Blake the situation of the chimney sweep is established in the first stanza where we are given vital information about his upbringing and background:
When my mother died I was very young,
And my father sold me while yet my tongue
Could scarcely cry "weep! weep! weep! weep!"
So your chimneys I sweep, and in soot I sleep.
So we can tell that since his mother died when he was very young and his father then sold him into slavery, his life has been one of constant drudgery and hard work. Note how "weep" is made to rhyme with "sweep" and "sleep" in the last two lines, emphasising the pain and hardship of his position. The internal rhyme in the last line also shows how endless his work is by emphasising the relationship between his work and his "sleep" - he has no chance to have a break and sleeps literally caked in soot because he is working all day.
However, the rest of the poem goes on to try and state that all those who face poverty and oppression will be compensated with endless joy in heaven - a sentiment that the partner of this poem devastatingly undercuts.