Natural gas is still a fossil fuel, so it doesnt produce particulates, and has less NOx and SOx, but you still get large amounts of CO2 which is bad for global warming. Also, the fact that natural gas costs 20 times as much per BTU as uranium makes it a pretty foolish alternative. Even when you consider lifetime costs where you add in the high price of nuclear plants natural gas is still several times more expensive than nuclear.
Natural gas and nuclear plants do NOT fill the same niche, nuclear plants are base load plants which produce pwoer 24/7 very cheaply, but they have to run at high capacity factors 90+% to acheive maximum profitability. Natural gas plants on the other hand are peaking plants and produce power only when it is in very high demand. Where I worked this summer at TVA the combined cycle natural gas plants were getting 8-10% capacity factors, and the combustion turbines were less than 1%. That measn that a 1000MWe combustion turbine only averages 10MW, whereas a 1000MWe nuclear plant would average 900MW. However, you need both types of plants for the grid to function correctly. Also, for the last 15 years natural gas plants are all anyone has been building, so there are currently way to many of them. Building more nuclear plants allows companies to create a diversified production shceme which is better suited to the demand curve than it currently is in many areas where natural gas plants are used as base load plants because they face less opposition than coal or nuclear plants during construction.
No single source can produce all our power needs, wind gets pretty iffy if its over 10% of capacity, base load nuclear and coal probably should be around 50-60% combined, and hydro and natural gas can be used as peaking units along with older less efficient coal units.