Erling Haaland took his tally Premier League strikes of the season.
At a Glance
Pep Guardiola's side take first place in the top flight following their victory.
Erling Haaland brings his total to 19 strikes in the league this season.
The Dutch midfielder scores another for City before half-time.
Erling Haaland found the net on two occasions to help Manchester City defeat beleaguered West Ham United for a fifth consecutive Premier League win.
This result sent them to the league's summit, a single point ahead of Arsenal, who have a chance to retake top spot in their evening fixture.
West Ham, under Nuno Espirito Santo, needed a positive start but found themselves behind inside five minutes when Haaland scored at the second attempt.
The Hammers were cut open with ease and Haaland somehow sent a free header wide soon after scoring.
The Dutchman added a second shortly before the break, collecting Haaland's pass before driving forward and hammering a finish into the roof of the net.
After the interval, the London side tried to claw something back but their forward could only find the side netting and dragged another shot wide.
The striker's brace then ensured a miserable festive period for the Hammers, who are stuck inside the relegation zone.
It marked City's seventh straight win in every competition and the team are starting to put together the kind of run that often leads to major silverware.
Haaland's early goal and second-half finish continued an remarkable campaign where he has scored 19 league goals.
After Reijnders slammed in the second goal, the outcome seemed certain as City extended their impressive unbeaten run against sides in the relegation zone.
The goalkeeper saved from two City players late on, but City's commanding win means they have not lost against West Ham in two decades of league meetings.
For the West Ham fans, any hope of a shock triumph was extinguished almost immediately of the start.
Nuno's side have not enjoyed a boost from a coaching change, winning just two and now winless in their past six league fixtures.
Consequently, they sit in 18th place, three points from safety, with the deficit potentially widening as other teams have games in hand.
Other than a couple of opportunities for Jarrod Bowen and a set-piece, they offered little trouble for the opposition goalkeeper.
Historically, West Ham have survived relegation when outside the bottom place at Christmas, providing a glimmer of hope for this season.
Pep Guardiola's men: They enjoy a week off before returning to action with a trip to Nottingham Forest.
Nuno's team: The Hammers host Fulham at their home ground on the same day.
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