Yes it is but your problem will be parking when you get there. Unless you have made arrangments to park somewhere, don't even think about using a car in Central London. It will be much easier - and faster - to use 'the tube' ie London Underground, even from Heathrow.
You have to use the Piccadilly Line of London Underground which serves all of Heathrow's Terminals.
About one hour and 15 minutes.
There are no direct trains between London City Airport and Heathrow.
London Underground serves Heathrow Airport from three different stations: Heathrow Terminals 1,2,3; Heathrow Terminal 4 and Heathrow Terminal 5. These stations are all on the Piccadilly line which runs through Central London. However, the travel-card does not cover you to travel on Heathrow Express which I believe is what you are referring to.
Heathrow airport serves the city of London, England.
By the title of the airport, London Heathrow Airport, it serves London and south-east England.
Once you arrive I would take the London Underground. The Piccadilly line runs from all Heathrow terminals to the centre of London. Get a Tube Map from an Underground station, then you can find your way anywhere.
about 30 min, 16 miles ( 27km)
The driving distance from London Heathrow Airport to London City Airport is 37 miles.
London
Heathrow Gatwick and London City Airport.
The are 5 airports that serve London London Heathrow (LHR) London Gatwick (LGW) London Luton (LTN) London Stanstead (STN) London City (LCY) The biggest of the 5 is London Heathrow, it is not only the busiest airport in the United Kingdom but within the top 10 busiest airports in the world. London Gatwick handles most holiday charter flights for the London area and indeed the Home Counties.