In general, a vehicle that uses all natural gas is much more efficient and produces less emissions than a gasoline powered car. In fact there are many all natural gas vehicles on the road now. There is a natural gas refueling station down the road from my house.
Natural gas is already in a gaseous state when its injected into the engine and ignited. Gasoline has to be vaporized to some extent when its injected into your engine and ignited. When you start your car up in the morning, you have what's known as a cold start. This is a key point in time when substantial emissions are produced by your car. Natural gas eliminates cold starts and emissions. It also burns hotter and more completely than gasoline.
The primary problem with an all electric vehicle is that battery technology isn't there to make it feasible. An electric motor is more efficient than a gas burning motor. If the technology was there for all electric vehicles it would be more efficient and produce less overall emissions to burn the gas in a power plant and produce electricity to charge batteries because power plants are more efficient than millions of small engines. Electric vehicles are ideal but not now currently realistic.
So how can we realistically (technologically and economically - from an engineering standpoint) improve the efficiency and therefore energy use of millions of smaller cars. We use a hybrid vehicle which incorporates a hydrocarbon fuel (like gasoline or natural gas) engine and a small electric engine combined. Remember the goal is to improve the efficiency of millions of smaller vehicles. The hybrid car that I designed used natural gas and only a very very small battery pack (and by the way the same type of batteries in your car now - lead acid). So the amount of energy and time needed to recharge the batteries is low. Regenerative braking aids in recharging.
You do need some electrical energy produced from a power plant or other source. But the quantity compared to what would be required for an all electric vehicle is substantially less because your only dealing with a fraction of the batteries. Quiet possibly in the near future a couple of solar panels might be all you need. So with a hybrid car you have substantially improved efficiency and fuel use in millions of small cars with a minimal increase in electrical demand.